Enhanced virulence of mpox virus clade Ib over clade IIb in the CAST/EiJ mouse model

The recently emerged clade Ib of mpox virus (MPXV) is spreading rapidly across Central and West Africa raising concerns about its potential virulence. Similar to clade IIb lineage B.1, which was responsible for the 2022 global outbreak, clade Ib exhibits sustained human-to-human transmission and a pattern of APOBEC3-associated genomic mutations. Here, we show that clade Ib displays enhanced long-range spreading in cell culture compared to clade IIb. Additionally, using the CAST mouse model, we show that clade Ib exhibits a higher level of virulence compared to the markedly attenuated clade IIb. Clade Ib leads to significant weight loss and high mortality in animals following both intraperitoneal and intranasal challenge. Histopathological analysis revealed more severe and extensive lung lesions in clade Ib-infected animals, accompanied by a broader distribution of viral antigens. Moreover, clade Ib, unlike IIb, disseminated efficiently to internal organs. These findings indicate that clade Ib MPXV has not undergone attenuation after human-to-human transmission to the extent observed in clade IIb and underscore the need for surveillance and preparedness against new emerging MPXV lineages.